structural engineers were stunned when the twin towers of the World Trade Center crashed to the ground on Sept. 11. How could these buildings have collapsed? Numerous causes have been looked at including fireproofing, height, impact of the planes, heat from the fire from spilled fuel, and the shell and core of the building itself.

The twin towers were part of a seven-building complex designed by architect Minoru Yamasaki that covered eight city blocks. An 800 x 400-ft foundation box, 65-ft-deep and with 3-ft-thick retaining walls, was under more than half the complex, including the twin towers and the adjacent hotel. The complex was completed in phases beginning in 1970 . The 1.8-million-sq-ft Seven World Trade Center, constructed in the mid-80s, also had a steel moment frame from the seventh story up. (ENR) Each of the twin towers stood more than 415 meters tall, and the buildings' floors enclosed more than one acre of office space. Seen from above, the 110-story twin towers were approximately squares, 209 feet on a side, with 59 columns on each face. The core, containing the elevators, stairwells and mechanical equipment, consisted of a rectangular arrangement of 47 heavier columns. The core columns carried about 60% and the exterior columns 40% of the towers' weight, which totaled 276,000 tons each above the plaza level.

Steel-framed buildings, such as the World Trade Center towers, are encased in cementitious fireproofing to protect framing members -- typically to provide structural integrity for up to three hours. But the fire loads that govern the amount of applied fireproofing are based on requirements for fires fed by materials found in offices such as paper, furniture and drapes, and not by volatile jet fuel.

The buildings, which for a while in the 1970s were the world's tallest, didn't have the traditional skyscraper's skeleton. They were designed to withstand the impact of a Boeing 707, which was the largest commercial aircraft at the time the buildings were designed. An exterior skeleton of closely spaced steel columns along with steel columns ringing a core of elevator shafts, air ducts and stair enclosures held up the towers. "No building before or since has put so much rentable space so high up in the air," says Leslie Robertson, the structural engineer for the towers. (Insight on the News -- Dec. 2001)

On September 11th, 2 Boeing 767s crashed into the World Trade Center, one into the North tower at about the 90th floor and penetrated approximately 150 feet into the building, while the other slammed into the South tower at about the 60th floor. The second jet proceeded to blast a hole through the adjacent side of the building. The impact shattered each of the aircraft, spraying jet fuel throughout the building. It is estimated that each of the 200 ton jets were traveling at least 300 miles per hour and carrying 9,000 gallons of jet fuel. Each impact severed many floors of the towers vertical support columns, presumably disabling the sprinkler system. An explosion of jet fuel created a fire with flames as hot as 2000 degrees Fahrenheit. Abolhassan Astaneh-Asl, a professor at the University of California at Berkeley, said preliminary evidence indicates the structures withstood the impacts of the planes. (CNN -- Oct. 2001) Astaneh-Asl, 53, led a team that studied damage to the San Francisco-Oakland Bay Bridge after the 1989 Bay Area earthquake, and has investigated methods to make buildings bomb-resistant.

Many have questioned why the 1,362-foot tall South tower, failed much as one would expect a felled tree, but the 1,368-foot tall North tower, similarly hit but at a higher floor, telescoped, failing vertically, rather than falling over. A question remaining is why the twin towers appeared to have collapsed in such different ways.

The American Institute of Steel Construction has said that there is much speculation regarding what caused the twin towers to collapse after the large, fuel-laden passenger jets flew into the buildings. (Robertson-2001) Engineers around the world believe the collapses occurred as a result of a combination of events, including the plane crashes and explosions that destroyed part of the structure and the ensuing fire, which progressively weakened the remaining structure. The subsequent collapses might have occurred, AISC said, when the weight of the structures above the points of impact exceeded the reduced load-carrying capacity of the underlying buildings.

When the World Trade Center towers succumbed to an inferno of jet fuel, they fell almost vertically, confining much debris to the trade center site. Fires and the rain of concrete and steel destroyed or seriously damaged the five other, smaller towers of the complex. Inspections of 195 neighboring buildings showed that most avoided structural damage, although all need months of repair and cleaning. (ENR -- 2001) "When these towers came down, there was an air burst out to the sides that punctured the skin of many surrounding buildings," carrying soot and ash, explains Ron Hamburger, director of the National Council of Structural Engineers Associations. (Terrell -- 2001)

The twin towers, framed in structural steel, had exterior moment frames with 14-in. steel box columns spaced 39 in. On center. The configuration created a complete tube around the building. The central steel core carried gravity loads only. The exterior tube provided lateral resistance. Horizontal steel trusses spanned 60 ft from the exterior wall to the core. Concrete on metal deck completed the floor diaphragm. Each tower contained about 100,000 tons of steel and 4 in. Of concrete topping on the 40,000-sq-ft floors, according to Henry H. Deutch, assistant to the chief structural engineer for construction manager Tishman Realty & Construction Co. Inc., New York City, during the construction of the WTC. (ENR -- 2001)

As the fires burned, the structural steel on the breached floors and above would have softened and warped because of the intense heat, say sources. Fireproofed steel is only rated to resist 1,500 to 1,600 Fahrenheit, while the blaze sent the fire temperature to well over 2,000 Fahrenheit. (Gold -- 2001) The structure began to warp and weaken at the top of each tower from the inferno. The frame, along with concrete slabs, furniture, file cabinets, and other materials, became an enormous, consolidated weight, which could no longer be supported. It was inevitable that it eventually crushed the lower portions of the frame below. The floor diaphragm, necessary to brace the exterior columns, lost its connection to the exterior wall. When the stability was lost, the exterior columns buckled outward, allowing the floors above to drop down onto floors below, overloading and failing each one as it went down. (Post-2002) Each failure rapidly led to a larger and more serious one until the buildings plummeted with incredible fury, their upper floors striking the ground at an estimated 120 miles per hour. One Pittsburgh-area structural engineer called the collapse "the zipper effect" for the way the floors came down upon each other. Another compared steel to a strand of spaghetti, which is rigid when uncooked yet when that spaghetti (steel) is heated, it loses its modulus elasticity and can bend. As the steel weakened, gravity took over and the weight of the upper floors caved in on the lower floors, causing what could be termed a self-implosion.

The role of the fireproofing was only one of a range of issues, including the part played by the jet fuel, which needed continued scrutiny. The inquiry into how and why the buildings fell has focused on the effect of the raging fires that swept through multiple floors of both towers in the aftermath of the plane attacks. Until now, most experts have said that the fire, fed by thousands of gallons of jet fuel carried by the planes, softened steel columns and the lightweight steel trusses holding up individual floors

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